Stereoscopic display of visual scenes requires that a separate two-dimensional image of a scene be supplied to each eye of the observer, wherein each image is a member of a stereo pair that depicts the scene from a different perspective viewpoint. The observer's visual system fuses the images, providing a realistic three dimensional impression. A number of known systems require the observer to wear glasses, goggles or similar devices to supply separate images to each eye. For many applications, however, such devices are inconvenient or impractical.
Autostereoscopic three dimensional displays direct separate two-dimensional images of a scene to each eye of the observer without requiring the observer to wear any device. An optical system superimposes the two-dimensional images on a viewing plane or screen, while directing the light from each image such that each eye of an observer perceives a different two dimensional image. If the two images are stereoscopically related perspective views of the scene, the observer will perceive a realistic three dimensional impression of the scene. This approach works with as few as two views, but in this case the observer must seek out the position where each eye perceives the correct image. A preferred embodiment employs more than two stereoscopically related scene images directed such that there is an array of viewing positions wherein the observer perceives a stereoscopic image pair. Further, when the observer moves side-to-side within the array of positions, his eyes will transition from image to image and he will experience a life-like change in the scene perspective. A wide array of viewing positions also allows multiple observers to view the same scene. Alternatively, each observer may be shown a different scene in applications such as video games. In all cases the systems are very intuitive to use: the observers simply approach the screen and perceive a 3-D image when they are in position to look through the array of viewing positions.
Such multiple view autostereoscopic displays typically comprise a planar array of holographic pixels or “hogels” on a screen wherein each hogel emits two or more directed light beams, wherein the brightness and color of each light beam may be dynamically varied by an image control system. An observer's eye positioned within a given light beam perceives that hogel as a pixel with the color and brightness unique to that beam, and does not perceive the beams emitted in other directions. Other hogels in the planar array emit light beams that are also directed at the observer's eye such that in aggregate the observer perceives an array of pixels that form a two dimensional image. At a different position the observer's eye is within a different set of light beams and perceives an array of pixels forming a different two dimensional image on the same hogel plane. The hogel array therefore provides apparatus capable of displaying dynamic two dimensional images that change with the observer's eye position, and in combination with an image control system may be used for autostereoscopic display and other visual effects. Several prior art means of forming a hogel array are known, and each has its advantages and drawbacks.
Spatially multiplexed flat panel dynamic autostereoscopic displays based on an angularly selective lenticular screen overlaid on a liquid crystal pixel array display are a commercially available implementation of the hogel principle. Multiple pixels under each lenticel each project light beams toward the observer in a different direction, providing the required variation of content with the observer's eye position. They have the disadvantage of dividing the resolution of the underlying liquid crystal display among the multiple two dimensional views, with the result that the perceived resolution is significantly lower than for comparable two dimensional displays. Time multiplexed flat panel dynamic autostereoscopic displays based on an angularly variable backlight illuminating a liquid crystal pixel array have been proposed. In these systems a sequence of two dimensional images is displayed within each video frame time interval, each image backlit from a different angle, to form multiple directed light beams in each hogel. The observer's persistence of vision provides the perception of a continuous image at each eye position. While full resolution of the liquid crystal display is preserved, the required display switching speed is increased by a factor equal to the number of light beam directions and the time average illumination and perceived brightness of each two dimensional image are decreased.
Multiple projector autostereoscopic displays where a separate projector is used for each set of hogel light beams provide full resolution and brightness at a normal frame rate switching speed are known in the prior art, and promise high quality image display without the compromises of spatially or time multiplexed displays. They typically focus the multiple projectors on a common screen such that each projector provides light from a different angle for each point on the screen to form the hogel array, and utilize controlled amounts of Gaussian diffusion in the screen to expand the projector light beams to avoid dark zones in the images or viewing zones. By its nature, however, Gaussian diffusion results in non-uniform illumination and causes overlap and double imaging between adjacent virtual apertures. WO 2014/070641 by this inventor describes novel projection screens for multiple projector autostereoscopic displays, and is incorporated by reference in its entirety. In particular, screens according to WO 2014/070641 utilize refractive optics rather than Gaussian diffusion to expand the projector light beams, largely eliminating non-uniform illumination and overlap and double imaging. Prior art projected displays of all types, however, require a significant amount of physical volume that increases with screen size in front of the screen for front projection or behind the screen for rear projection to accommodate the projector light paths, limiting application flexibility compared to flat panel displays.
The autostereoscopic displays discussed thus far have hogels that provide horizontal perspective variation only (X-only), resulting in lack of perspective change with vertical changes in the observer's position that can appear unnatural. Adding vertical perspective variation (X-Y) has been proposed in the literature, but is primarily confined to hologram-like non-dynamic images in practice since it greatly increases the system performance requirements to display the additional perspective views. Spatially multiplexed and time multiplexed X-Y displays face increased image quality compromises, and multiple projector X-Y displays require a large increase in the number of projectors that in the past would have been difficult in practice because of physical projector size and cost. The situation is changing. Multiple projector displays, including X-Y displays are becoming an increasingly practical proposition, driven by current and anticipated future reductions in the size, cost and energy consumption of commercially available video projectors. Such projectors include picoprojectors small enough and low enough in cost to be integrated into cellular telephones, exemplified by a projector described in US 2010/0066921 by EI-Ghoroury et al. assigned to Ostendo Technologies, Inc. The Ostendo projectors, shown in FIG. 13 of US 2010/0066921, comprise a multicolor laser array image source, drive circuitry, and projection lens in a small module that requires no additional optical elements or illumination sources. Further, the image source is fabricated on a silicon wafer providing potentially low cost production as well as the stable image geometry desirable in applications in which multiple projectors are combined such that each contributes a portion of an overall image. US 2010/0066921 is incorporated in its entirety by reference.
Display image resolution expectations of users are also increasing. Commercially available two dimensional high end flat panel and projected displays offer 4000 lines, establishing a significant performance target for X-only and X-Y autostereoscopic displays. Such resolution, however, is out of reach using current picoprojectors in conventional configurations wherein each projector illuminates the full screen area.
A need clearly exists for multiple projector autostereoscopic systems based on small, low resolution projectors that provide high resolution images having both horizontal and vertical perspective variation and having relatively small physical volume.